Microtubule dynamics change dramatically during the cell cycle, from the relatively stable interphase microtubules to the very dynamic mitotic spindle. Studies with cell-free extracts of Xenopus eggs have shown that the interphase-mitosis transition of microtubule dynamics involves regulation of catastrophe rate, which is induced by phosphorylation reactions mediated by cdc2 kinase. However, both catastrophe factors and substrate proteins for these kinases that may play key roles in the change of microtubule dynamics remain to be identified. To quantitate the catastrophe activities and purify the potential catastrophe factors, I have developed a simple pulsed-chase assay to measure the catastrophe rate. Using this assay, I have shown that the catastrophe factors are active in both mitotic and interphase extracts, but the activities seem modulated during the cell cycle. The objective of this proposal is to purify the catastrophe factors, study the roles of such factors in the regulation of microtuble dynamics and spindle morphogenesis. To characterize the microtubule dynamics, both biochemical assay and real time video microscopy will be used. Since microtubule plays vital role in the cell cycle regulation, therefore regulation of proliferation, this project is directly related to cancer research.